Investigation of Sediment Entrainment Using Dual-Phase, High-Speed Particle Image Velocimetry

To advance the understanding of the phenomenon of brownout, a dual-phase flow environment induced by a laboratory-scale rotor hovering above a sediment bed was studied using high-speed flow visualization and particle image velocimetry (PIV). The high frame rate of the camera, combined with particle recognition and tracking software, permitted a first-principles understanding of the temporal evolution of the rotor wake in ground effect simultaneously with the processes of sediment entrainment and transport by the rotor flow. Near-wall PIV measurements showed that the convecting blade tip vortices produced large excursions in the surface boundary layer flow The highest sediment entrainment levels were observed to occur within the wake impingement zone, which resulted from the local increases in groundwash and upwash velocities produced by the tip vortices. Once entrained, significant quantities of sediment were intermittently suspended, or trapped, by the vortex-induced upwash field. Specifically, both the local groundwash and upwash fields were shown to strengthen significantly during the viscous merging of adjacent blade tip vortices. This latter mechanism proved fundamental in defining the concentration of suspended sediment, as well as the maximum height to which sediment could be transported. Sediment particles reaching sufficient heights were observed to recirculate into the rotor wake and convect back toward the ground. This process caused sediment ejection by means of bombardment or splash. The classical process of saltation bombardment was also visualized for larger particles whose inertia prevented them from being suspended in the vortical flow. While providing new insight into the time- and length-scales associated with sediment transport by a rotor wake, the observations made here also raise questions about the efficacy of uplift and entrainment models currently being used for brownout simulations.